Nanocrystalline materials for soft magnetic applications

Nanocrystalline materials comprise a unique class of magnetic materials in which the extrinsic magnetic properties such as low core losses, high permeability and almost zero magnetostriction (lambda(s)) can be controlled by reducing the grain size to be less than the exchange interaction length. The ability to control these properties and the availability of saturation magnetization of up to 1.4 T make the nanocrystalline soft magnetic alloys highly attractive to applications engineers. In the present study, the frequency dependences of AC coercivity (H-c) and initial permeability (mu(i)) of nanocrystalline strip in as-cast and annealed condition were investigated. In the case of optimized material, DC coercivity H-c < 1 Am-1 and lambda(s) < 10(-6) were obtained. The effect on mu(i) of annealing these materials was more marked at frequencies < 10 kHz. The frequency dependence of mu(i) in the case of net-shaped cores consolidated from nanocrystalline flakes was dramatically improved by the application of an inter-flake insulation layer. Phosphate coating, when applied to the flakes as an insulation layer prior to their consolidation, resulted in mu(i) greater than or equal to 1000 at 100 kHz. A semi-quantitative model is proposed to explain this effect. Some potential applications of the insulated net-shaped cores are also described.